1. Field of the Invention
The present invention relates generally to fracturing a well formation, and more particularly to propellant assemblies for creating fractures in a well.
2. Background
Techniques for perforating and fracturing a formation surrounding a borehole are known in the art. Generally, some techniques for perforating and fracturing a formation to stimulate production include the steps of: 1) penetrating a production zone with a projectile; and 2) pressurizing the production zone to initiate and propagate a fracture—either by igniting a propellant device or hydraulically.
Godfrey et al., U.S. Pat. No. 4,039,030, describes a method using a propellant to maintain the pressure caused by a high explosive charge over a longer period. The high explosives are used to generate fractures while the role of the propellant is to extend these fractures. In accordance with this technique, the casing must be perforated prior to ignition of the high explosives and propellant as the high explosives are used exclusively to fracture the formation but not to perforate the casing.
Ford et al., U.S. Pat. No. 4,391,337, describes integrated perforation and fracturing device in which a high velocity penetrating jet is instantaneously followed by a high pressure gas propellant. In essence, a tool including propellant gas generating materials and shaped charges is positioned in a desired zone in the borehole. The penetrating shaped charges and propellant material are ignited simultaneously. The high pressure propellant material amplifies and propagates the fractures initiated by the shaped charges.
In Hill, U.S. Pat. No. 4,823,875, the well casing is filled with a compressible hydraulic fracturing fluid comprising a mixture of liquid, compressed gas, and proppant material. The pressure is raised to a level about 1000 psi greater than the pressure of the zone to be fractured by pumping fluid downhole. The gas generating units are simultaneously ignited to generate combustion gasses and perforate the well casing. The perforated zone is fractured by the rapid outflow of an initial charge of sand-free combustion gas at the compression pressure followed by a charge of fracturing fluid laden with proppant material and then a second charge of combustion gas.
Dees et al., U.S. Pat. No. 5,131,472, and Schmidt et al., U.S. Pat. No. 5,271,465, each concern overbalance perforating and stimulation methods, which employ a long gas section of tubing or casing to apply high downhole pressure. Fluid is pumped downhole until the pressure in the tubing reaches a pressure greater than the fracture pressure of the formation. A perforating gun is then fired to perforate the casing. Because the applied pressure is enough to break the formation, fractures propagate into the formation. The gas column forces the fluid into the fractures and propagates them.
Couet et al., U.S. Pat. No. 5,295,545, describes an overbalance technique for propagating a fracture in a formation by driving a liquid column in the wellbore into the formation by activation of a gas generator (e.g., compressed gas or propellant).
Passamaneck, U.S. Pat. No. 5,295,545, discloses a method of fracturing wells using propellants which burn radially inward in a predictable manner—including a computer program for modeling the burn rate of the propellant to determine a suitable quantity and configuration of the propellant for creating multiple fractures in the surrounding formation.
Snider, et al., U.S. Pat. No. 5,775,426, and Snider, et al., U.S. Pat. No. 6,082,450, each describe an apparatus and method for perforating and stimulating a subterranean formation using a propellant secured to the outside of a perforating gun containing shaped charges or a carrier.